The invention relates to a circuit for rapidly switching, in a television receiver, at least three video sources which are connectable to a video processing stage providing the display of a picture, comprising at least two cascaded change-over switches, the first of which is connected directly to the video processing stage and the second is connected to the first, and whose switching action is controlled by means of a plurality of control signals.
The circuit according to the invention has for its object to select sources for forming a video picture on the screen of a television receiver. In addition to the internal main source which forms the demodulation stage output of the television receiver itself, it is possible to connect to the latter second auxiliary source constituted by external peripheral apparatus, more specifically a magnetoscope of which a special socket for the connection of peripheral apparatus, the so-called peritelevision socket is provided whose terminals can be connected to the video processing stage of the television set via change-over switches.
A circuit providing the necessary switching actions is described in European Patent Application No. 0 046 108. This circuit utilizes more specifically, for the control of the change-over switches a control signal applied to a terminal of the peritelevision socket. In the absence of this signal, if no special "audiovisual" button is pushed, the inverters are in the position in which they select the internal source.
There may be one or a plurality of third sources. Processes of transmitting magazine pages, for example the method known as Antiope, form a third video source. Similarly, numerical encoding methods make it possible to transmit, for example, sub-titles.
This third source is generally constituted by a decoder which forms an internal auxiliary source of the television receiver, which source can be selected independently of the other two sources. Consequently a change-over switch capable of switching in three directions is required.
Two sources can be used simultaneously to produce an inset, that is to say the alphanumerical characters coming from a source are inserted in an image coming from another source. To that end, at appropriate instants, the main picture is interrupted during the line scan to be replaced by any character element to be displayed, and the picture thereafter continues in the normal way until the subsequent character.
This requires very fast switching circuits, as, for example, a character element whose width is one hundredth of the overall width of a picture has a duration of not more than approximately 0.5 microseconds.
Fast-acting electronic switches are then necessary.
Such a fast three-way electronic change-over switch can be realized with the aid of a cascade arrangement of two fast two-way change-over switches. A black level aligning circuit is associated with each way of these change-over switches to ensure that the black level of the several sources are identical.
This switch assembly is comparatively complicated and costly.